backend/tests: Add blackbox tests from Kotlin JVM

Added tests from testData/codegen/box directory. There are blackbox tests
in other directories and they are to be added.

backend.native/tests/external/codegen/blackbox/dataClasses/multiDeclaration.kt

@@ -0,0 +1,7 @@
+data class A(val x: Int, val y: Any?, val z: String)
+
+fun box(): String {
+    val a = A(42, null, "OK")
+    val (x, y, z) = a
+    return if (x == 42 && y == null) z else "Fail"
+}